Submersible or line shaft centrifugal pumps are currently used in a number of applications. One common application involves the use of a centrifugal pump to remove ("strip") a liquid cargo from a marine vessel, such as a tanker ship.
It is a common practice to mount marine cargo pumps such that the impeller rotates about a vertical axis and is driven by a vertical shaft extending to and above the surface of the liquid cargo. A pipe is provided to carry the pumped liquid, and this pipe also serves to enclose and support the vertical shaft. The impeller and its associated inlet apparatus are located as close to the bottom of the tank as possible to minimize the sump depth.
Another requirement for marine cargo pumps is that liquid inside the pump and attached piping system be prevented from draining back into the tank whenever the pump is stopped. Drained back liquid presents a severe cleaning problem when the type of cargo in a specific tank is to be changed, because the tank has to be cleaned of the old product and whatever amount that remains is difficult to store and finally dispose of. In addition, in marine applications a short down time for washing is required.
It is also important that the pump be stopped the moment it loses suction due to an empty tank. If the pump is not stopped immediately, it begins to operate dry, causing damage to its internal bearings. Depending on the type of cargo, the pump may lose suction a number of times during the cargo stripping operation as liquid slowly drains to the tank sump. Thus, the pump must be easily restartable after a loss of suction.
It is known in the art to provide a separate butterfly valve attached to the bottom of a marine cargo pump. The prior art butterfly valve is an off-the-shelf purchased item which has not been optimally designed for use with centrifugal pumps in marine cargo stripping operations.
The prior art butterfly valve and pump combination suffers from a number of drawbacks. The total axial length of the pump and butterfly valve combination is greater than necessary due to the fact that the butterfly valve assembly is a purchased addon item and not integrally built into the pump. In addition, the amount of time required to close the prior art butterfly valve is relatively long, because the valve must rotate around its axis. A portion of the butterfly valve thus has to move opposite the flow direction. Dirt or other contamination can easily jam the butterfly valve so that it cannot easily be turned. Butterfly valves are notorious for being difficult to tightly seal in dirty liquid environments. Matching problems between flow through the butterfly valve and flow to the pump are common. Inlet flow to the impeller is disturbed, which deteriorates the efficiency of the impeller.